This invention relates to catalysts for oxidative dehydrogenation of alkanes and a process for producing olefins using said catalysts. More specifically, the invention relates to the catalysts which are suitable for use in vapor phase oxidative dehydrogenation of C2-C5 lower alkanes (hereinafter occasionally referred to simply as xe2x80x9clower alkanesxe2x80x9d) in the presence of molecular oxygen to produce corresponding olefins, and a process for oxidizing and dehydrogenating lower alkanes at vapor phase with molecular oxygen to produce corresponding olefins at high yields, with the use of said catalysts.
Lower olefins are starting materials of important industrial products: Le., ethylene, for ethylene oxide, acetaldehyde, acetic acid and the like; propylene, for acrolein, acrylic acid, propylene oxide, polypropylene and the like; and isobutene, for methacrolein, methacrylic acid, methyl tert.-butyl ether and the like. Demands for those products invariably increasing recently and their prices running higher, development of low cost production process of these lower olefins is desired.
As a production process for lower olefins, in particular, propylene and isobutene, simple dehydrogenation process of lower alkanes is recently reduced to industrial practice. However, this process is subject to an essential problem that it is incapable of giving high conversion due to the equilibrium limitation and furthermore requires high temperatures. Still in addition, deterioration of the catalyst within a short period is inavoidable in said process, which necessitates frequent regeneration of the catalyst using a switch converter or the like. In consequence, plant construction costs and utility costs for running the process are high and, depending on the conditions of factory location, it is unprofitable and its industrial application at the present time is restricted.
Whereas, attempts to produce lower olefins from lower alkanes through oxidative dehydrogenation which is free from the limitation by equlibrium have been made since long, and various catalyst systems therefor have been proposed. Among those known, there are Coxe2x80x94Mo oxide catalyst (U.S. Pat. No. 4,131,631), Vxe2x80x94Mg oxide catalyst (U.S. Pat. No. 4,777,319), Nixe2x80x94Mo oxide catalyst (EP 379,433 A1) CeO2/CeF3 catalyst (CN 1,073,893A), Mgxe2x80x94Mo catalyst [Neftekhimiya (1990), 30(2) 207-10], V2O5/Nb2O5 catalyst [J. Chem. Commun. (1991) (8) 558-9], rare earth vanadates catalyst [Catal. Lett. (1996), 37, (3,4), 241-6] and B2O3/Al2O3 catalyst [ACS Symp. Ser. (1996), 638 (Heterogeneous Hydrocarbon Oxidation) 155-169). Those known catalysts, however, invariably show very low level oxidative dehydrogenation performance and are far short of industrially practicable level.
We also have disclosed for the purpose catalyst containing Cr or Mo, Sb and W as the essential components (2000-037624A-JP) and those containing Mn as the essential component (2000-037625A-JP), but catalysts exhibiting still higher activity level are desirable for industrial use.
An object of this invention is to provide novel oxidative dehydrogenation catalysts useful for vapor phase oxidative dehydrogenation of lower alkanes with molecular oxygen to produce corresponding lower olefins at high yields; and also to provide a process for producing from lower alkanes the corresponding olefins at high yields, by the use of said catalysts.
The above object of the invention can be accomplished by the catalysts which are characterized in that they contain Mn as the essential component and a crystal phase which is identified by the peaks appearing in X-ray diffraction spectrum (per Cuxe2x80x94Kxcex1 cathode) when diffraction angle 2xcex8 (xc2x10.3xc2x0) is at 32.9xc2x0, 55.2xc2x0, 23.1xc2x0, 38.2xc2x0 and 65.8xc2x0 (i.e., when the inherent crystal lattice spacing, d-values, are 2.72 xc3x85, 1.66 xc3x85, 3.84 xc3x85, 2.35 xc3x85 and 1.42 xc3x85), that is, the crystal phase corresponding to Mn2O3. Use of the catalysts in the occasions of vapor-phase oxidative dehydrogenation of C2-C5 lower alkanes enables production of lower olefins at high yields.
More specifically, in the invention said C2-C5 lower alkanes signify ethane, propane, n-butane, isobutane, n-pentane and isopentane. These lower alkanes may be used either singly or as a mixture of more than one kind.
According to the invention, from these lower alkanes the corresponding olefins can be prepared at high yields, more specifically, ethylene from ethane, propylene from propane, n-butene from n-butane, isobutene from isobutane, n-pentene from n-pentane, and isopentene from isopentane. The invention is particularly suitable for producing propylene from propane and isobutene from isobutane.
The catalysts according to the invention are characterized by containing Mn and a crystal phase identified by the peaks appearing in X-ray diffraction spectrum (per Cuxe2x80x94Kxcex1 cathode) when the diffraction angle 2xcex8 (xc2x10.3xc2x0) is at 32.9xc2x0, 55.2xc2x0, 23.1xc2x0, 38.2xc2x0 and 65.8xc2x0. In particular, those composed substantially of the elementary composition expressed by the following formula (1) are preferred:
MnaXbYcOxxe2x80x83xe2x80x83(1) 
in which Mn stands for manganese,
X stands for at least one element selected from the group consisting of Sb, W and Cr,
Y stands for at least one element selected from Re, Fe, Co, Ni, Nb, Ta, Ce, Zn, Tl, Li, Na, K, Rb, Cs, Mg, Ca, Sr and Ba,
and O stands for oxygen.
Of the catalysts represented by above general formula (1), those containing as the X component at least two elements selected from Sb, W and Cr, or all of those three elements are preferred. Among the preferred catalyst, those containing as the Y component one to four elements selected from the group consisting of Ni, Co, Nb, Ta, Ce, Li, Na and K are particularly advantageous.
Referring to the above general formula (1), a, b, c and x stand for the atomic ratios of Mn, X, Y and oxygen, respectively, and where when a is 1, b is 0.01-2, c is 0-2 and x is a numerical value determined by the extents of oxidation of those elements other than oxygen. Particularly recommendable catalysts are those in which, where a is 1, b is 0.05-1 and c is 0-1.
The oxidative dehydrogenation catalysts of the present invention may further contain refractory inorganic substances for the purpose of improving their activity level and physical durability. Suitable content of the refractory inorganic substances is 10-90% by weight of the whole catalyst containing the catalytically active ingredients. As such refractory inorganic substances, those known can be used, examples of which including silica, alumina, titania, zirconia, silica-alumina, silica-titania and silica-zirconia. In particular, silica, silica-alumina and titania are preferred, because they give higher yield of the object products. As the silica-alumina, those in which the ratio of the silica is at least 10% by weight but less than 100% by weight are suitably used.
The process for preparing the catalysts of the invention are not subject to critical limitations, so long as the catalysts containing the specified crystal phase are ultimately obtained. Whereas, depending on the kind of manganese source used, for example, combination of the preparation steps may become subject to certain limitations such as a need to modify the heat treatment conditions in the subsequent steps. Individual steps, however, are subject to no specific limitation but known methods can be applied. For example, the catalysts according to the invention can be prepared by the following steps: add at least one of antimony trioxide powder, aqueous solution of ammonium metatungstate and aqueous solution of chromium nitrate, to a slurry containing manganese (III) oxide powder; if necessary also add an aqueous solution or an oxide powder of a compound of at least one element selected from the group consisting of Ni, Co, Li, Na, K, Re, Fe, Nb, Ta, Ce, Zn, Tl, Rb, Cs, Mg, Ca, Sr and Ba; optionally further add a refractory inorganic substance such as silica, alumina or the like; stir and mix the resulting slurry for a prescribed period, heat and condense the system; dry the resulting paste at 80-300xc2x0 C.; grind down and shape the dried system; if necessary crush the shaped pieces to adjust their sizes; and re-dry them at 80-300xc2x0 C. or if necessary further fire them at 300-800xc2x0 C.
The above drying and firing may be performed in any kind of atmosphere, such as under high oxygen concentration, low oxygen concentration, in reducing atmosphere, inert gas (nitrogen, helium, argon and the like), or in vacuum. The catalysts of the invention may be contacted with the reaction gases containing alkanes and oxygen, after the drying at temperatures not higher than 300xc2x0 C., e.g., aforesaid range of 80-300xc2x0 C., without the high temperature firing such as the one at 300-800xc2x0 C. as above. In that occasion, the reaction may be initiated directly at a prescribed reaction temperature, or a reaction concurrently serving as a preliminary reaction may be carried out at a temperature higher than the prescribed reaction temperature. In that case, the catalyst""s activity variation may be observed during the initial stage of the reaction but it normally stabilizes within an hour. Compared to the case of high temperature firing the catalyst at temperatures not lower than 300xc2x0 C., e.g., at 300-800xc2x0 C., in an atmosphere other than the reaction gas, such direct treatment with the reaction gas as above tends to achieve improved activity and selectivity and, therefore, is especially preferred.
Starting materials to be used for the catalyst preparation are not critical, but any of nitrate, sulfate, oxide, hydroxide, chloride, carbonate, acetate, oxygen acid, ammonium salt of oxygen acid, etc. of each metal may be used.
As the Mn source, besides powders of various oxides thereof or molded products which are useful as they are, all of those which can be prepared by generally accepted means, such as manganese hydroxide slurries obtained upon treating an aqueous solution of, e.g., manganese nitrate, with aqueous ammonia or the like, coprecipitation products from aqueous solutions containing manganese compounds and compounds of other catalytically active elements, are useful. Of those, the starting materials containing Mn2O3, inter alia, manganese oxide composed substantially of Mn2O3 alone, are preferred. The X-ray diffraction spectrum of such a manganese oxide shows only the peaks identifying the crystal phase corresponding to Mn2O3.
An ultimately obtained catalyst must contain the crystal phase which is identified by the peaks appearing in its X-ray diffraction spectrum (per Cuxe2x80x94Kxcex1 cathode), where the diffraction angle 2xcex8 (xc2x10.3xc2x0) is at 32.9xc2x0, 55.2xc2x0, 23.1xc2x0, 38.2xc2x0 and 65.8xc2x0. (i.e., when d-values are 2.72 xc3x85, 1.66 xc3x85, 3.84 xc3x85, 2.35 xc3x85 and 1.42 xc3x85) (i.e., the crystal phase corresponding to Mn2O3). This can be accomplished by either using Mn2O3 as the Mn source and adopting such preparation conditions as will finally maintain the Mn2O3 phase, or by adopting such preparation conditions under which Mn2O3 phase will be formed during the preparation procedure, where the starting material does not contain Mn2O3. Preferred practice is to use Mn2O3 as the starting material and not to conduct a heat treatment at temperatures not lower than 300xc2x0 C.
When Mn oxide is used as the starting material, preferred specific surface area of said material ranges 0.5-10 m2/g. When the area exceeds this range, complete oxidation activity of resulting catalyst becomes intense and high partial selectivity cannot be obtained. Whereas, when the area is less than said range, catalytic activity is reduced.
As the Sb source, powders of oxides such as Sb2O3, Sb2O4, Sb2O5 and the like, Sb oxides as dissolved in aqueous tartaric acid, antimonic acid (antimony pentoxide hydrate) sol, and the like are conveniently used. Of those, antimonic acid sol or Sb2O3 powder are convenient, in respect of uniform catalyst preparation and performance of resulting catalyst. While antimonic acid sol is commercially available as various products, most of them are unsuitable as starting material for the catalyst because various stabilizers are incorporated therein. An antimonic acid sol which is obtained by passing an aqueous solution of potassium antimonate through a strongly acidic cation exchange resin for effecting ion-exchange is preferred because it is free of impurities.
As starting materials of other elements, use of water-soluble materials is generally preferred, while water-insoluble starting materials such as oxides may also be used depending on kind of the elements.
Form of use of those refractory inorganic substances is not subject to particular limitation, but it can be suitably selected from various forms such as shaped bodies, powder, gel and Sol, according to the individual use form of the catalyst.
The starting gas to be subjected to the vapor phase oxidative dehydrogenation reaction according to the present invention may, if necessary, contain a diluent gas, besides lower alkane(s) and molecular oxygen. As the source of molecular oxygen, air or pure oxygen is used. As the diluent gas, an inert gas such as nitrogen, helium or carbon dioxide, or steam is conveniently used. It is normally satisfactory to use 0.1-5 mols of molecular oxygen per mol of alkane.
The reaction conditions for carrying out the vapor phase oxidative dehydrogenation of the present invention are subject to no critical limitation. For example, the starting gas as described above is contacted with an oxidative dehydrogenation catalyst of the present invention under such conditions as: at a space velocity of 300-30,000 hrxe2x88x921 at a temperature between 250 and 650xc2x0 C.
While the reaction is normally conducted under atmospheric pressure, a reduced or elevated pressure may be used. Form of the reaction system again is not critical, which may be a fixed bed system, moving bed system or fluidized bed system. It may also be one-pass system or recycling system.